STR:

1. Open about:certificate (make sure to be on a profile that will make the "People" tab visible)
2. While the page loads, you'll see a blue line (from https://searchfox.org/mozilla-central/rev/f43cef551760b268a3d22eaddd0e76719acf2d88/toolkit/components/certviewer/content/components/certificate-section.css#15) for ~1-2 seconds until the page fully loads.

Note that this reproduces also on a new profile in which the about:certificate page won't have the People tab, but it's hard to see that. Will try to catch a screenshot.

(In reply to Itiel from comment #0)

Note that this reproduces also on a new profile in which the about:certificate page won't have the People tab, but it's hard to see that. Will try to catch a screenshot.

After a frame-by-frame testing, it's just that the bottom border vanishes really quickly. But it's still there if you look close enough.
On my main profile it's there for full 2-3 seconds though.

Seems like a minor visual issue :)

Set release status flags based on info from the regressing bug 1669857